1. Field of the Invention
The present invention relates in general to a speed changing gear device in an automatic transmission for motor vehicles such as automobiles and railway carriages, and more particularly to such a speed changing gear device which is equipped with two single-pinion type planetary gear units and one double-pinion type planetary gear unit.
2. Discussion of the Prior Art
A known speed changing gear device used in an automatic transmission for a motor vehicle has a plurality of suitably assembled planetary gear units, each of which has a sun gear, a ring gear, at least one planetary pinion meshing with the sun and ring gears, and a carrier rotatably supporting the planetary pinion or pinions. One of these gears and carriers (pinions) of the planetary gear units serves as an input element which is connected to an input member of the transmission, and another element of the gear device serves as an output element which is connected to an output member of the transmission, while other elements may be fixed to a stationary member such as a transmission casing. In this arrangement, a rotary motion of the input member of the transmission is transmitted to the output member, at different speed reduction ratios, for forward or reverse running of the vehicle.
The speed reduction ratios provided by the speed changing gear device as described above vary to a considerable extent, depending upon the manner of assembling or connecting the planetary gear units, the gear ratio of each planetary gear unit (i.e., number of teeth of the sun gear divided by number of teeth of the ring gear), and the number of the planetary pinions each planetary gear unit has. However, not all conceivable configurations of the planetary gear device are practically usable. Namely, the speed changing gear device having a series of planetary gear units can be practically adopted if the gear device satisfies various conditions or limitations, such as installation adaptability to the vehicle body, feasibility of production, and power transmitting and speed changing characteristics and performance. In other words, numerous types of planetary gear devices can be constructed by merely changing the combination and the gear ratios of the planetary gear units. Therefore, it is rather difficult to figure out the speed changing gear device which satisfies all the conditions indicated above required for an automatic transmission for a motor vehicle.
Various types of speed changing gear devices suited for use in the automatic transmission have been proposed under the situation as described above. For example, laid-open Publications Nos. 51-17767, 51-48062, 51-108168, 51-108170 and 51-127968 of unexamined Japanese Patent Applications disclose speed changing planetary gear devices having three planetary gear units.
In the speed changing gear device having a plurality of planetary gear units, however, the number of operating positions provided by the gear device and the speed reduction ratios of the respective operating positions are greatly variable, depending upon how the planetary gear units are connected to each other, which element of the gear device is connected to the input member of the transmission and which elements are made fixable to a stationary member. Practically, a suitable speed changing gear device for a vehicle transmission is selected based on the output characteristic of the engine to which the transmission is connected, and the type and required characteristics of the vehicle on which the transmission is installed. If the different types of motor vehicle require speed changing gear devices which are differently constructed in the gear train arrangement of the planetary gear units, as well as in the location of the clutches and brakes, the number of the types of the gear devices that should be prepared is as large as that of the types of the motor vehicles on which the gear devices are installed. This results in poor productivity of the gear devices due to increased time for designing and manufacturing thereof. Namely, there should be prepared many basic arrangements of planetary gear assembly, which cannot enjoy common manufacturing or assembling steps, and the productivity of the necessary line of gear devices is accordingly lowered.
As described above, the speed changing gear device having a plurality of planetary gear units is capable of establishing a plurality of speed reduction ratios, which largely differ depending on the manner of connecting the planetary gear units and the locations of the clutches and the brakes in the planetary gear assembly. Therefore, it is technically possible to determine the number of operating positions and the speed reduction ratios of the gear device as needed, by changing the numbers and locations of the clutches for receiving power from the engine and the brakes for holding stationary the selected elements of the gear device, while maintaining the same combinations of elements of the planetary gear units which are permanently fixed together, or connectable to each other by clutch means. In this way, the problem described above can be solved to some degree since the same basic arrangement of the planetary gear assembly may be used for different configurations of the gear devices. In this case, it is desirable that the basic arrangement of the planetary gear assembly is made relatively compact and small-sized, and easy to manufacture, and provides the speed reduction ratios which vary substantially in the form of a geometric progression, for reducing shifting shocks. It is also desirable that the basic arrangement provides a relatively wide range of speed reduction ratios, which includes a speed reduction ratio of "1" or lower as needed.
On the other hand, the conventional speed changing gear devices as disclosed in the above-identified publications are not able to establish an overdrive position whose speed reduction ratio is "1" or lower. Further, these publications do not show the manner of changing the arrangement of the clutches and brakes so as to change the operating positions provided by the gear devices. In the conventional gear devices, the speed reduction ratios of the operating positions do not always change in the form of a geometric progression, thereby unfavorably inducing the shifting shocks, which deteriorate the driving comfort of the vehicle.